For example, a noise-cancelling function in a headphone cancels ambient sound, such as noise in a train or an aircraft, leaking in the headphone, to provide a quiet audio-visual environment enabling concentration on music appreciation further.
Meanwhile, because the noise-cancelling function makes the ambient sound difficult to hear, a noise-cancelling headphone has been known for having an external-sound capturing function of capturing external sound from a microphone provided to the outer housing of the headphone, namely, a monitoring function, from the viewpoint of safety or for emergency response (e.g., refer to Patent Document 1). Such noise-cancelling headphones mostly achieve the external-sound capturing function with diversion of a microphone for noise cancelling installed in the external housing.
Such an external-sound capturing function is mostly included in mainly noise-cancelling headphones, and recently Bluetooth (registered trademark) headphones are mostly equipped with the noise-cancelling function. Therefore, the external-sound capturing function is added to many Bluetooth (registered trademark) headphones each attached with the noise-cancelling function.
A typical external-sound capturing function is mostly operated on or off with a dedicated switch (button) implemented in a headphone or a cable of the headphone, or the function may be operated with an operation of an application of a smartphone, for example, in a case where the headphone or the like has no switch.